The present invention relates to the formation of relatively large sheets of niobium tin or Nb.sub.3 Sn for use in superconducting magnets.
It is known to use superconducting cylindrical shields as a superconducting gradient shield in magnetic resonance imaging magnet coils. Such shields shield the magnetic fields resulting from pulsing of the imaging coils from interfering with the magnetic fields produced by the main magnet superconducting coils. Copending patent application by Bu-Xin Xu et al., Ser. No. 08/236,060, filed May 2, 1994, entitled "Superconducting Gradient Coil Shields in Magnetic Resonance Imaging Magnets" and assigned to the same assignee as the subject patent application, which patent application is hereby incorporated by reference provides a detailed description and understanding of the fabrication and use of superconducting sheets of relatively large size in a superconducting gradient shield for magnetic resonance imaging and the magnetic characteristics needed for such applications. However, superconducting materials are not commercially available in large sheets for use in forming a relatively large diameter superconducting gradient shield (hereinafter "SGS"). SGS utilize cylinders whose diameter may be in the order of 1 meter and whose length may be in the order of 3 meters.
While it is known to fabricate SGS out of Nb.sub.3 Sn and NbTi, the fabrication and resulting superconducting magnetic characteristics have proven to be less than completely satisfactory. Nb.sub.3 Sn reacted foil wound radially over a transverse layer or cylinder is found to shield quite well and have a minimized heating during magnetic resonance imaging (hereinafter "MRI") but provides a time constant which is less than desirable. On the other hand, NbTi sheet provides an acceptable time constant due to the sheet's geometry but exhibits a heating which is higher than desirable during MRI imaging due to a critical current density, JC, of 1.75.times.10.sup.-9 A/m.sup.2 at 4K and 2T in NbTi that is lower than that resulting from use of Nb.sub.3 Sn. Such heating is highly undesirable since it increases the rate of helium boiloff in an MRI helium cooled superconducting magnet.
However, Nb.sub.3 Sn exhibits a brittleness which makes it difficult to fabricate in large sheets, necessitating welding multiple sheets together, with the sheets being difficult to fabricate into the large cylinders required for SGS in superconducting magnets used in MRI applications. As a result, it has been recognized for some time that it would be desirable to have a material providing a combination of a long time construct, minimum heating during operation and ease of handling in the fabrication of relatively large SGS cylinders for MRI applications. Considerable research has been directed at obtaining such a material.